@Tav'ith'sava the problem with comparing this to P8P adventures is that those are on some level meant to leave space for the DM. They can't encompass everything and aren't truly complete until the players (including the GM) meet it and fill it. Meanwhile BG3 is an absolute shambles of a story that never actually holds together when it's a game that doesn't have the same degree of openness to allow players to truly mold it.
As to the point of this thread, I doubt Mephistopheles was ever involved in the plot. It's possible though that Zariel had a larger place in the narrative and perhaps it's through her actions they were able to steal the prism. Like so many story beats in this game, stuff about the Hells are both weirdly prominent but at the same time frustratingly tangential.
